XE NMR AND AR QUASI-EQUILIBRIUM SORPTION OF COKED H-Y ZEOLITE

Acidic, HY zeolite was partially coked with propylene at 250 and 475°C. Standard micropore volume determination by N2 physical sorption indicates that nearly all of the coke was confined to the micropores with little change in the larger mesopores. Xe NMR chemical shifts increased linearly with a decrease in the micropore volume for the coke samples. However, initial coke deposition on HY increased the Xe chemical shift beyond the changes predicted by the decrease in micropore volume. The increase in chemical shift due to coke was also found to be dependent on the aromaticity of the coke. Coke formation at 250°C (65% spz carbon) increased the chemical shift by 6.5 ppm while coke formed at 475°C (>95% spz carbon) increased the chemical shift by 8.5 ppm. The size of the pore apertures, i.e., the opening to the alpha cage, was determined by low-pressure Ar adsorption isotherms. As the level of coke increased, the size of the alpha cage aperture decreased by as much as 0.07 nm. A bimodal distribution in the pore apertures was clearly resolved on some catalysts at higher coke levels. One pore size was characteristic of apertures essentially free of coke (0.72 nm); while, the other, smaller aperture was more severely restricted due to coke deposits (0.58 nm). The inability of Xe NMR to detect two pore environments observed by Ar sorption may result from rapid exchange of Xe between the two sites. This implies that the distance between the two pore environments is small and that the two pore environments are uniformly distributed throughout the catalyst particle. © 1991.